For certain applications, there is a need for a broadband, high power communication system. For example, in military applications a broad bandwidth is required for secure spread spectrum communication and high power is required for long range. High power broadband communication systems require high power broadband antennas. Often these antennas have an input impedance that does not match the desired transmitter or receiver with which it is used. In such circumstances, baluns can be used to transform the impedance of the antenna to the impedance of the transmitter or receiver, or to convert between an unbalanced signal and a balanced signal. When large bandwidths are desired, coaxial baluns are often used.
Simple signal sources have two terminals, a source terminal and a return terminal, where most commonly a ground plane is used for the return path. The ground plane return simplifies circuit wiring, as a single conductor and the ground plane below form a complete signal path. The voltage on the ground plane is then the reference for this signal. Often this is referred to as an “unbalanced circuit”, or “single-ended circuit”. In such “unbalanced circuits” when wires cross or run parallel with one another, there can be undesired coupling.
One method for reducing such coupling is to use two wires, one carrying the signal, the other carrying the return signal, with no ground plane return path. With AC signals, either wire can be considered to carry the signal, and the other to carry the return signal. To minimize coupling to other circuits, it is highly desired that the signal current flowing in the two wires be exactly the same, and 180-degrees out of phase. That is, all of the return current for one wire of the pair is carried by the other wire, and the circuit is balanced. This guarantees that no return current is carried by the ground plane. In practice, such perfectly balanced, or differential, currents are only a theoretical goal.
An amplifier that uses balanced or differential input and output connections is less likely to have oscillations caused by input and output signals coupling, and less extraneous noise introduced by the surrounding circuitry. For this reason, practically all high gain operational amplifiers are differential. A “balun” is a coupling device that converts an unbalanced source to a balanced one, and vice versa. Sometimes a balun is made with nearly complete isolation between the balanced terminals and ground. Sometimes a balun is made with each balanced terminal referenced to ground, but with equal and opposite voltages appearing at these terminals. These are both valid baluns, but in one case, the unbalanced voltage encounters high impedance to ground, making unbalanced current flow difficult, while in the other, any unbalanced current encounters a short circuit to ground, minimizing the voltage that enters the balanced circuit. Microwave baluns can be either of these types, or even a mixture of the two. In any case, one could connect 2 equal unbalanced loads to the 2 balanced terminals, with their ground terminals connected together to ground. Ideally, the unbalanced signal input to the balun would be equally distributed to the two unbalanced loads. Thus, a balun may be used as a power divider or combiner, where the two unbalanced loads or sources connected to the balanced terminals would be operating 180-degrees out of phase.
At microwave frequencies, it is very difficult to fabricate well balanced circuits, as small parasitic elements can unbalance the signals. A well balanced power divider or combiner that operates over a wide microwave bandwidth is thus a very important component, and one that supplies differential, 180-degree out-of-phase outputs is most desirable because of its independence from currents flowing in the ground plane.